1. Field of the Invention
The present invention relates to a method and system for deriving a travel distance of an object which travels, such as vehicles, and more particularly to the method and system which employ a combination of measurement of the travel distance based on a reception of position-measuring electric waves and measurement of the travel distance based on a stand-alone manner.
2. Description of the Related Art
At present, there are known navigation systems functioning as position-measuring devices for various vehicles such as automobiles, airplanes or ships. The navigation systems are generally constructed such that they display a position mark representing the current traveling position of a driver's vehicle at a determined point on a map including the current traveling position of the vehicle in a superimposed manner, so as to guide the driver in determining travel routes to a desired destination. In terms of the principle of operation, vehicle navigation systems are generally classified into two types: stand-alone type navigation systems and GPS (Global Positioning System) type navigation systems.
In the former navigation systems, a travel direction and a travel distance of a vehicle are determined on the basis of outputs of a velocity sensor and an angular velocity sensor both installed in the vehicle, the determined data regarding the direction and distance are added to data of a preset reference position to obtain a current traveling position, and data of the calculated current traveling position are used to place a mark in the superimposed manner, on a map displayed on a monitor.
In the latter navigation systems, position-measuring electric waves are received from a plurality of GPS satellites which have been launched into space, data of the received electric waves are used to calculate a current traveling position of a driver's vehicle through the three dimensional or two dimensional measurement technique, and data of the calculated current traveling position are used to place a position mark on a necessary map displayed by a monitor display screen in the superimposed manner.
Still, a recent tendency shows that a third type, called a hybrid type vehicle navigation system, having functions of both the standalone and the GPS types has been in common use.
The foregoing vehicle navigation systems permit the user (driver) to recognize a current traveling position of his vehicle in conjunction with a map around the current traveling position. As a result, if the driver is not very familiar with the area, he can get enough navigation information to reach a desired destination.
In addition to the foregoing method utilizing the position-measuring electric waves, the foregoing vehicle navigation systems may employ another method for calculating the travel distance of vehicles. Specifically, in such another method, a pulse signal outputted in response to the rotation of wheels (i.e., rotation of wheel shafts) is used to convert a travel velocity (km/h) measured using the position-measuring electric waves into velocity per second. The velocity per second thus obtained is divided by the number of pulses of the pulse signal, thus obtaining a travel distance per pulse (m/pulse) (hereinafter, referred to as "unit travel distance"). Multiplying the calculated unit travel distance by the total number of pulses detected during the travel produces a travel distance in a stand-alone manner.
In actual applications, however, calculating the travel distance obtained based on the position-measuring electric waves may pose some problems regarding its accuracy. For example, a variety of factors including changes in the number of GPS satellites usable for measuring positions and/or changes in reception states of position-measuring waves owing to surroundings around a current traveling position of a vehicle sometimes prevent travel distances from being calculated in high accuracy. Still, when vehicles run through shadowed places, such as tunnels, where position-measuring electric waves are hardly received, it frequently occurs that distances which a vehicle has moved during travel with receiving no position-measuring electric waves are not counted, thereby increasing errors of travel distances if such occasion happens. Thus, in the case of the foregoing hybrid-type vehicle navigation system, in normal operation, the unit travel distance is updated and held to calculate travel distances based on not only the number of pulses of a pulse signal produced in response to vehicle's travel but also the unit travel distance. If required, the unit travel distance held at that time will be corrected for error-less calculation of travel distances using the position-measuring electric waves.
Such configuration, where travel distances calculated in the stand-alone manner using regularly updated unit travel distances are corrected with a travel distance calculated based on position-measuring electric waves, provides higher accuracy in obtaining travel distances of vehicle.
However, there is another problem in calculation of travel distances based on the above-mentioned pulse signal. This problem, resulted from the fact that the unit travel distance is affected and changed by changes in travel states of a vehicle, is that accurate calculation of travel distances is not always guaranteed.
Various drawbacks concerning the errors of travel distances due to the travel state changes will be explained in detail for automobiles regarded as vehicles. The first drawback is concerned with seasonal kinds of tires. Winter tires and summer tires are different in diameter and air pressure from each other. Hence, for example, when driving in winter season, exchanging summer tires for winter tires inevitably produces a different unit travel distance. In this situation, the unit travel distance after the tire exchange will be updated based on the unit travel distance calculated based on the summer tires. Since there is a large difference in the unit travel distance between summer tires and winter tires, it takes excessive time for updating the unit travel distances. As a result, there are produced large errors of the unit travel distances during such excessive time interval (until it begins to obtain the unit travel distance suitable for the winter tires). The first drawback is therefore that proper calculation of travel distances cannot be made.
The second drawback is also directed to tire exchanges. Even if unit travel distances suitable for the winter tires are obtained after being exchanged for the winter tires, there is some possibility that travel states are changed by, for example temporarily applying chains to tires or slipping of tires. In this case, the update of unit travel distances will be made under chain-applied tires or slipped tires. Although such applying chains or slipping is a short-term event, unit travel distances discontinuously changes after removing the chains or end of the slip. Therefore, until a suitable unit travel distance for the winter tires is again obtained, the unit travel distances with larger errors will last, thereby providing the second drawback that travel distances cannot be calculated properly.
The third drawback arises when automobiles go backward. When automobiles normally go backward by setting the shift lever to the reverse position in the transmission, a backward travel signal indicative of the backward movement (i.e., the signal for turning on the back lamps) is produced. The foregoing conventional vehicle navigation systems monitor the logic level of the backward travel signal (usually the logic level "H" in backward movement), and does not add to the travel distance of the system (travel distance for forwarding) the travel distance obtained during the logic level of "H". In contrast, some automobiles are designed to generate the backward travel signal of "L" when they move backward. If trying to install the foregoing conventional navigation systems into both the two types of automobiles, there arises a drawback that a travel distance calculated in backward movement is automatically added to the travel distance which had been calculated in forward travel, because the systems misinterpret the backward travel signal of "L" as being the signal in forward movement. This also leads to the third drawback that is apt to increase errors in calculated travel distances.
Further, there is a fourth drawback relating to noise filters. Conventional vehicle navigation systems have an LPF (Low Pass Filter) for removing noise in a radio frequency band of the above-said pulse signal. However, different types of automobiles may produce various kinds of pulse signals different in the number of pulses generated per rotation of wheel shafts. For example, when the frequency band of an LPF is set to well be applied to automobiles where a higher number of pulses per rotation of wheel shafts is generated, the LPF cannot practically remove noise existing in a radio frequency band of a pulse signal generated by an automobile whose number of pulses per rotation of wheel shafts is lower. In consequence, noise would be included into the pulse signals of automobiles which generate lower numbers of pulses of in pulse signals, thereby providing the fourth drawback that reluctantly raises errors in travel distances calculated based on pulse signals into which noise is included largely.